# Thread: Manually calculating gear ratio inputs

1. Junior Member
Join Date
Jan 2015
Posts
0

## Manually calculating gear ratio inputs

Hi,

I have a G2X (red). Although I can monitor my speed via GPS, I am interested in setting
up the gear ratios in order to see which gear I'm in. I know I can do it "automagically"
in Setup2, but would rather do it the manual way.

Have done a search, but can't find a "method" to to calculate them properly.

I have a 944S and my current gear ratios are:

1 - 3.500:1
2 - 2.059:1
3 - 1.400:1
4 - 1.034:1
5 - 0.829:1

I'm running 235/50/15 tires which gives me a circumference of 73 inches (if I measured correctly)

This might be a really basic question.....but.....How do I calculate the ratios for proper entry? Or do
I simply enter them as they are?

Thanks,

Jim  Reply With Quote

2. Junior Member
Join Date
Jul 2012
Location
IL
Posts
11

## Re: Manually calculating gear ratio inputs

I get 76 in for your tire diameter (=pi()*(235*0.5*2/25.4+15)). If you actually measured 73 in, then use that. The best number to use is the revs/mi number that the tire manufacturer provides. If you don't know this number, see if Tire Rack has this number for your tires. If you still can't gets revs/mi, then you can calculate it.

revs/mi = 5280*12/Tire Circumference

Once you have the revs/mi number for the tire, you can calculate the required ratio as follows.

ratio = GearX*GearFinal*revs_per_mi/60

Where Gear final is your final gear and GearX is the gear ratio you are calculating for.  Reply With Quote

3. Junior Member
Join Date
Jan 2015
Posts
0

## Re: Manually calculating gear ratio inputs Originally Posted by mc-integra111
I get 76 in for your tire diameter (=pi()*(235*0.5*2/25.4+15)). If you actually measured 73 in, then use that. The best number to use is the revs/mi number that the tire manufacturer provides. If you don't know this number, see if Tire Rack has this number for your tires. If you still can't gets revs/mi, then you can calculate it.

revs/mi = 5280*12/Tire Circumference

Once you have the revs/mi number for the tire, you can calculate the required ratio as follows.

ratio = GearX*GearFinal*revs_per_mi/60

Where Gear final is your final gear and GearX is the gear ratio you are calculating for.
Excellent. Although I'm totally math challenged, I think I can figure this out now. TireRack did have the revs/mi for my tire
so that makes it easy.

Thanks for the help!!!!

J  Reply With Quote

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